1. Field of the Disclosure
Embodiments disclosed herein relate generally to wellbore fluids used in downhole operations. In particular, embodiments disclosed herein relate to wellbore fluids possessing decreased abrasiveness.
2. Background
In the exploration for natural resources, such as oil and gas, drilling assemblies are used to penetrate earth formations of varying hardnesses. The drilling assembly typically comprises an earth-boring drill bit mounted on the lower end of a drill string, which is rotated by rotating the drill string at the surface or by actuation of downhole motors or turbines, or by both methods. When weight is applied to the drill string, the rotating drill bit engages the earth formation and proceeds to form a borehole along a predetermined path toward a target zone. The mechanism of the engagement of the earth formation is dependent on the type of drill bit used in the assembly, and may be any of shearing, crushing, gouging or abrading mechanisms. These downhole tools are therefore constantly subjected to erosive forces whilst engaging the earthen formation.
Drilling fluids are pumped down the drill string and across the surface of the drill bit where it cools and cleans the drill bit surface. Various fluids are used when drilling or completing a well, and the fluids may be used for a variety of reasons. Common uses for well fluids include: lubrication and cooling of drill bit cutting surfaces while drilling generally or drilling-in (i.e., drilling in a targeted petroliferous formation), transportation of “cuttings” (pieces of formation dislodged by the cutting action of the teeth on a drill bit) to the surface, controlling formation fluid pressure to prevent blowouts, maintaining well stability, suspending solids in the well, minimizing fluid loss into and stabilizing the formation through which the well is being drilled, fracturing the formation in the vicinity of the well, displacing the fluid within the well with another fluid, cleaning the well, testing the well, transmitting hydraulic horsepower to the drill bit, fluid used for emplacing a packer, abandoning the well or preparing the well for abandonment, and otherwise treating the well or the formation.
Proper movement of the wellbore fluids, used to remove the rock cuttings and cool the exposed surface of the cutting tool, is important for the proper function and performance of these cutting tools. For example, the cutting face of a diamond impregnated bit typically includes an arrangement of recessed fluid paths intended to promote uniform flow from a central plenum to the periphery of the bit. The fluid paths usually divide the abrasive layer into distinct raised ribs with diamonds exposed on the tops of the ribs. The fluid provides cooling for the exposed diamonds and forms a slurry with the rock cuttings or drill solids. The slurry must travel across the top of the rib before reentering the fluid paths, which contributes to wear of the supporting material.
In general, drilling fluids should be pumpable under pressure down through strings of drilling pipe, then through and around the drilling bit head deep in the earth, and then returned back to the earth surface through an annulus between the outside of the drill stem and the hole wall or casing. Beyond providing drilling lubrication and efficiency, and retarding wear, drilling fluids should suspend and transport solid particles to the surface for separation and disposal. In addition, the fluids should be capable of suspending additive weighting agents (to increase specific gravity of the mud), generally finely ground barites (barium sulfate), and transport clay and other substances capable of adhering to and coating the borehole surface.
Drilling fluids are generally characterized as thixotropic fluid systems. That is, they exhibit low viscosity when sheared, such as when in circulation (as occurs during pumping or contact with the moving drilling bit). However, when the shearing action is halted, the fluid should be capable of suspending the solids it contains to prevent gravity separation. In addition, when the drilling fluid is under shear conditions and a free-flowing near-liquid, it must retain a sufficiently high enough viscosity to carry all unwanted particulate matter from the bottom of the well bore to the surface. The drilling fluid formulation should also allow the cuttings and other unwanted particulate material to be separated from the liquid fraction after transport to the surface.
There is an increasing need for drilling fluids having the rheological profiles that enable wells, especially deep or horizontal wells, to be drilled more easily. Drilling fluids having tailored rheological properties ensure that cuttings are removed from the wellbore as efficiently and effectively as possible to avoid the formation of cuttings beds in the well which can cause the drill string to become stuck, among other issues. There is also the need from a drilling fluid hydraulics perspective, particularly with respect to equivalent circulating density, to reduce the pressures required to circulate the fluid. This helps to avoid exposing the formation to excessive forces that can fracture the formation causing the fluid, and possibly the well, to be lost. In addition, an enhanced profile is necessary to prevent settlement or sag of the weighting agent in the fluid, if this occurs it can lead to an uneven density profile within the circulating fluid system, which can result in loss of well control, such as due to gas/fluid influx, and wellbore stability problems, such as caving and fractures.
Fluid characteristics required to meet these challenges include, for instance, that the fluid must be easy to pump, requiring the minimum amount of pressure to force the fluid through restrictions in the circulating fluid system, such as bit nozzles or down-hole tools. In other words, the fluid should have the lowest possible viscosity under high shear conditions. Conversely, in zones of the well where the flow area is large, velocity of the fluid is low, where there are low shear conditions, or when the fluid is static, the viscosity of the fluid should be as high as possible in order to prevent settlement, suspend, and transport the weighting material and drilled cuttings. However, it should also be noted that the viscosity of the fluid should not continue to increase under static conditions to unacceptable levels. Otherwise, when fluid circulation is regained, this can lead to excessive pressures that can fracture the formation or alternatively can lead to lost time if the force required to regain a fully circulating fluid system is beyond the limits of the pumps.
Wellbore fluids must also contribute to the stability of the well bore, and control the flow of gas, oil or water from the pores of the formation in order to prevent, for example, the flow or blow out of formation fluids or the collapse of pressured earth formations. The column of fluid in the hole exerts a hydrostatic pressure proportional to the depth of the hole and the density of the fluid. High-pressure formations may require a fluid with a specific gravity of 3.0 or higher. As such, weighting agents for drilling fluids are mainly selected based on the density they provide for maintaining wellbore stability.
Additionally, when a drill bit wears out, or fails as a bore hole is being drilled, it is necessary to withdraw the drill string for replacing the bit. The amount of time required to replace a bit and return the drill string to functional use is essentially lost from drilling operation. This time can become a significant portion of the total time for completing a well, especially if the well depths are great. This excessive wear due to abrasion therefore also results in increased maintenance and production cutbacks. Abrasive wear on downhole equipment can incur large costs.
Reducing the abrasive wear of downhole tools may reduce downtime for repair. This would prolong the time spent drilling and therefore increase the efficiency and cost-effectiveness of the drilling operation. Accordingly, there exists an increasing need to maximize the service life of downhole tools.